The present invention relates generally to the field of pneumatic actuators, and more specifically to pneumatically powered actuators having hydraulic actuation control.
Robotic applications have increased steadily with advancing technology. Installation of manipulators to automate assembly line tasks has become commonplace because of the increased productivity, reliability, and cost savings which can be realized through their use. The military is also interested in the application of robotics to missions where they may decrease risks to personnel and significantly enhance the probability of mission completion.
Manipulator actuators must generally satisfy a large number of functional criteria. Desirable qualities of a manipulator may include a high strength-to-weight ratio, high torque throughout translation, quick response to signal orders, smooth reversibility, high stiffness with low power consumption when idle, and positioning accuracy. Traditional choices for actuators are electric motors and either hydraulic or pneumatic actuators. Each of these have noted advantages and disadvantages.
Hydraulic actuators typically provide large force capability and significant power-to-weight ratios in fixed installations. They are suitable in applications requiring high force generation, stiffness, and precision control in tasks such as drilling or other machine tool operations. They are able to operate in dirty, abrasive, or wet environments and tolerate temperature extremes well. They may be safely used in explosive environments and generally provide a higher speed of response than electric motors. However, the disadvantages of hydraulic actuators include high cost due to the requirement for precision parts, contamination susceptibility, fluid leakage, and support components that are both bulky and heavy. Some operating fluids present a fire hazard.
Pneumatic actuators are primarily found in simple manipulation schemes. Typically, they provide uncontrolled motion between mechanical stop and are mainly used where point-to-point motion is required. They are simple to control and have relatively low operating costs. However, the compressibility of the actuating fluid eliminates any possibility of system stiffness. Pneumatic actuators, therefore, do not provide accurate position control between the limits of stroke.
Electric actuators are relatively low in cost and interface well with drive circuitry. They are used to power manipulators in low strength, precision applications such as in the manufacture of electronic circuit boards. They are easy to control, provide good torque, speed, and continuous power output performance, and operate quietly. However, their heavy power consumption makes them unsuitable for use in mobile applications where the energy usually is provided by onboard batteries. Unless electric actuators directly drive the manipulator joints, they must operate at high speed through long, backlash-prone gear trains. Furthermore, some sort of brake is required to hold position if power use is to be minimized, and for safety reasons in the event of a power failure. Electric actuators are usually not as rugged as hydraulic and pneumatic actuators and cannot operate in dirty, abrasive, wet, and corrosive environments unless they are sealed.
Performance requirements for a specific application can be difficult or impossible to achieve with conventional actuators from any single one of these categories. For example, one type of application may demand an actuator that is powerful, yet is light and rugged and which provides a well-controlled, precision actuation that can be locked at any intermediate position with no standby energy drain.
U.S. Pat. No. 3,779,135 by Sugimura, Dec. 18, 1973 discloses an air cylinder in which a piston rod and attached piston are reciprocated by air pressure alternately supplied to air chambers separated by the piston. A single-acting hydraulic cylinder is formed in a bore extending through the rod and piston of the air cylinder. This hydraulic cylinder has a hollow plunger secured at its one end to an end plate of the air cylinder. An inside bore of the plunger is communicated through a liquid conduit to an accumulator that includes a bladder precharged with a gas. An air valve controls the air supply to the air cylinder upon electrical actuation of left and right solenoids. A liquid valve is provided in the liquid conduit. The valve is opened during the time when its operating solenoid is energized and closed when the solenoid is deenergized. When the air valve is opened, the gas filled bladder introduces sponginess into the hydraulic system due to compressibility of the gas. Therefore, this mechanism is incapable of providing precise velocity control which is necessary for many robotic applications. In fact, this device is intended to only to provide a hydraulic braking to an air actuator. Therefore, there is a need for a pneumatically actuated cylinder which can provide positive braking as well as precision velocity control of the actuating link.
Accordingly, it is an object of the present invention to provide a pneumatic actuator having precision actuating rod velocity control without any sponginess. Another object of the present invention is to provide an a pneumatically powered actuator that can be positively locked in a given position. A further object of the present invention is to provide a hydraulically controlled pneumatic actuator that does not require an external accumulator.